Free-running circuit arrangement

ABSTRACT

The present invention relates to a free-running circuit arrangement for operating a load (La), having at least one switching element (T 1,  T 2 ), a freewheeling diode that is connected in an anti-parallel fashion relative to the main current direction of the at least one switching element (T 1,  T 2 ), a load circuit (L 3,  C 6,  C 5 ) and a control resonant circuit (L 2,  C E,T1 , C E,T2 ) which comprises at least one control inductor (L 2 ) and at least one self-capacitance (C E,T1 , C E,T2 ) of the at least one switching element (T 1,  T 2 ), the at least one switching element (T 1,  T 2 ) having a control electrode, a working electrode and a reference electrode, there acting between the control electrode and the working electrode a capacitance (C M,T1 , C M,T2 ) that is coupled to the control resonant circuit (L 2,  C E,T1 , C E,T2 ) in such a way that energy is fed into the control resonant circuit (L 2,  C E,T1 , C E,T2 ) by the charging and discharging current (I CM,T1 , I CM,T2 ) of this capacitance (C M,T1 , C M,T2 ), the circuit arrangement having no components for feeding energy into the control resonant circuit by electromagnetic coupling.

TECHNICAL FIELD

[0001] The present invention relates to a free-running circuitarrangement for operating a load, having at least one switching element,a freewheeling diode that is connected in an anti-parallel fashionrelative to the main current direction of the at least one switchingelement, a load circuit and a control resonant circuit which comprisesat least one control inductor and at least one self-capacitance of theat least one switching element.

BACKGROUND ART

[0002] Such a generic free-running circuit arrangement is disclosed inDE 195 48 506. The principle in accordance with which this circuitoperates can be gathered from FIG. 1 there: an inductor L2, in this casethe lamp inductor, arranged in the load circuit is coupled to twoauxiliary windings HW1 and HW2, the auxiliary winding HW1 being arrangedin the control resonant circuit of a first switching element T1, and theauxiliary winding HW2 being arranged in the control resonant circuit ofa second switching element T2. The oscillation of the circuitarrangement is maintained by electromagnetic feedback of energy from theload circuit into the respective control resonant circuit.

[0003] The disadvantage of this circuit arrangement consists in thatwound items are very expensive and therefore substantially predeterminethe costs of a circuit arrangement in addition to transistors.

[0004] It is therefore the object of the present invention to developthe generic free-running circuit arrangement in such a way that it ispossible to maintain an oscillation without this requiringelectromagnetic coupling of components of the load circuit to componentsof the control resonant circuit.

DISCLOSURE OF THE INVENTION

[0005] This object is achieved according to the invention by virtue ofthe fact that in the case of the generic free-running circuitarrangement the at least one switching element has a control electrode,a working electrode and a reference electrode, there acting between thecontrol electrode and the working electrode a capacitance that iscoupled to the control resonant circuit in such a way that energy is fedinto the control resonant circuit by the charging and dischargingcurrent of this capacitance, the circuit arrangement having nocomponents for feeding energy into the control resonant circuit byelectromagnetic coupling.

[0006] The invention is based on the finding that the energy required tomaintain the oscillation can be fed to the control resonant circuit viaa capacitance. An expensive, frequently individually fabricatedtransformer is therefore no longer necessary.

[0007] A further advantage of the circuit arrangement according to theinvention consists in that the circuit arrangement can be implementedwith a smaller number of components.

[0008] In a particularly advantageous embodiment, one switching elementhas a Miller capacitance, the capacitance acting between the controlelectrode and the working electrode on the switching element comprisingthe Miller capacitance of the at least one switching element. Millercapacitances occur, for example, in field effect transistors. Dependingon the design of the circuit, it can be possible that the Millercapacitance of the at least one switching element is alone sufficient tosupply energy to the control resonant circuit for maintaining theoscillation. Should the Miller capacitance not suffice alone, anadditional discrete capacitance can be connected in parallel with it.

[0009] It is therefore particularly advantageous to tune the controlresonant circuit and the Miller capacitance of the at least oneswitching element to one another in such a way that the oscillation ofthe control resonant circuit is maintained solely by the charging anddischarging currents of the Miller capacitance of the at least oneswitching element. This results in a further saving on components.However—as mentioned—it can also be provided to connect an additionalcapacitance in parallel with the Miller capacitance of the at least oneswitching element, the oscillation of the control resonant circuit beingmaintained by the charging and discharging currents of the Millercapacitance and of the additional capacitance of the at least oneswitching element.

[0010] The input capacitance present between control and referenceelectrodes of the at least one switching element can be used as theself-capacitance of the latter.

[0011] It is preferred for the circuit arrangement according to theinvention to have a first and a second switching element, the first andthe second switching elements being of complementary design and beingcoupled to a common control resonant circuit. The complementary designof the switching elements permits the use of a common control resonantcircuit for both switching elements. It is preferred in this case foreach switching element to have a control electrode, a working electrodeand a reference electrode, the control electrodes being connected to oneanother with the formation of a first tie point, and the referenceelectrodes being connected to one another with the formation of a secondtie point, the control inductor being coupled between the first and thesecond tie point.

[0012] However, it can also be provided that the circuit arrangement hasa first and a second switching element that are of the same type, thefirst switching element being coupled to a first control resonantcircuit, and the second switching element being coupled to a secondcontrol resonant circuit. Again, it is preferred for each switchingelement to have a control electrode, a working electrode and a referenceelectrode, the reference electrode of the first switching element beingconnected to the working electrode of the second switching element, andthe respective control inductor being coupled between the respectivecontrol and reference electrodes of the respective switching element.This implementation has the advantage that two switching elements ofidentical type can be used, the electric performance thereby also beingidentical.

[0013] The first and the second switching element are preferablyarranged in a half-bridge arrangement.

[0014] The load is preferably an illuminating means, preferably alow-pressure discharge lamp. However, it is also possible to use thecircuit for other types of loads.

[0015] For the case in which the input capacitance of the at least oneswitching element is unfavorably dimensioned for implementing thecircuit arrangement, it is preferred to connect a discrete supplementarycapacitance in parallel with the input capacitance of the at least oneswitching element. This option yields further degrees of freedom fordimensioning the control resonant circuit.

[0016] The load circuit preferably has a series tuned circuit with aninductor, a capacitance connected in parallel with the load and at leastone decoupling capacitance. The inductor is preferably dimensioned as acurrent-limiting and resonance inductor.

[0017] The at least one switching element is preferably a bipolartransistor or an MOS field effect transistor. For the case in which thecircuit arrangement is implemented with the aid of at least one MOSfield effect transistor, the body diode of the MOS field effecttransistor can implement the freewheeling diode connected in ananti-parallel fashion. A discrete diode is to be provided asfreewheeling diode in the case of an implementation of the at least oneswitching element as bipolar transistor.

[0018] Further advantageous developments of the invention are defined inthe subclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] An exemplary embodiment of the invention is described in moredetail below with reference to the attached drawings, in which:

[0020]FIG. 1 shows a circuit diagram of a ballast for operating anilluminating means, having a free-running circuit arrangement accordingto the invention;

[0021]FIG. 2 shows a detail of the ballast in FIG. 1, transistorcapacitances being illustrated by dashes;

[0022]FIG. 3 shows the detail of FIG. 2 with additional capacitances forincreasing the input capacitances and the Miller capacitances of thetransistors;

[0023]FIGS. 4a to 4 g show a time sequence of the current flows as theyoccur in the detail in accordance with FIG. 2; and

[0024]FIG. 5 shows the time profile of some variables of the detail inFIG. 2.

BEST MODE FOR CARRYING OUT THE INVENTION

[0025] The ballast illustrated in FIG. 1 has a supply connection L, Nthat is connected via a fuse Si to a rectifier comprising four diodesD1, D2, D3, D4. The DC voltage smoothed by a capacitor C1 is madeavailable via a filter having an inductor L1 and a capacitor C2 as whatis termed intermediate circuit voltage U_(Z) of the free-running circuitarrangement. Because of the voltage U_(Z), the capacitor C3 is chargedvia the resistors R1 and R2. The circuit also comprises two MOS fieldeffect transistors T1, T2, the drain terminal of the transistor T1 beingconnected to the positive terminal of the capacitor C2, the drainterminal of the transistor T2 being connected to the negative terminalof the capacitor C2. The source terminals of the two transistors T1, T2are connected to one another with the formation of a tie point VP1, thetie point VP1 being connected to the positive terminal of the capacitorC3. The negative terminal of the capacitor C3 is connected to frame viaa diode D5 and a resistor R3, the diode D5 and resistor R3 ensuring thatthe capacitor C3 is discharged during operation of the circuitarrangement, that is to say when the transistor T2 is turned on, so thatthe diac that is connected between the negative terminal of thecapacitor C3 and the two gate terminals of the two transistors T1, T2 isnot ignited in an interfering fashion during operation of the circuitarrangement. The series connection of an inductor L2 and a resistor R4is arranged between the tie point VP1 and the interconnected gateterminals of the transistors T1, T2. Arranged on the one hand betweenthe drain terminal of the transistor T1 and the tie point VP1 is theseries connection of a resistor R5 and a capacitor C4, the seriesconnection being connected in parallel with the series connection of acapacitor C5, the illuminating means La and the lamp inductor L3, acapacitor C5 being connected in parallel, for its part, to theilluminating means La.

[0026]FIG. 2 shows a detail of the ballast of FIG. 1, the transistorcapacitances being illustrated by dashes. Arranged between therespective gate terminal and the respective drain terminal is therespective Miller capacitance C_(M,T1) or C_(M,T2), the respective inputcapacitance C_(E,T1) or C_(E,T2) is arranged between the respective gateterminal and the respective source terminal, while the respective outputcapacitance C_(A,T1) or C_(A,T2) is arranged between the respectivedrain terminal and the respective source terminal.

[0027] For the case in which the transistor input capacitances C_(E,T1),C_(E,T2) and/or the Miller capacitances C_(M,T1) or C_(M,T2) are notsufficiently large to maintain oscillation, FIG. 3 shows a preferredoption for their enlargement. Here, a capacitance C_(MZ) is connected inparallel with the Miller capacitances C_(M,T1), C_(M,T2), while acapacitance C_(EZ) is connected in parallel with the input capacitancesC_(E,T1), C_(E,T2).

[0028]FIG. 4 shows snapshots of the current flows in the circuit detailof FIG. 2, in time sequence. Corresponding to this, the time profile ofsome variables of the circuit detail of FIG. 2 is illustrated in FIG. 5with reference to the voltages illustrated in FIG. 2.

[0029] Before considering the normal operation, that is to say themaintenance of an oscillation of the free-running circuit arrangement,the first aim is to explain the processes that unfold until aself-maintaining oscillation is produced. The starting point is a DCvoltage present across the capacitor C2 as a result of which thecapacitor C3 is charged via the resistors R1, R2. Because of the voltagerise across C3, the voltage present at the diac also rises. If thelatter voltage exceeds the trigger threshold of the diac, the latterturns on and, as a result, a negative voltage pulse reaches the two gateterminals of the complementary transistors T1 and T2. This charges thecapacitances C_(M,T1), C_(E,T1), C_(E,T2), while the capacitanceC_(M,T2) is discharged. The current flow illustrated in FIG. 4a is setup.

[0030] Because of the negative voltage pulse at the gate electrodes ofthe two transistors, which is effected by the ignition of the diac, thetransistor T2 is opened, see the current flow I_(ST2) in period 4 b inFIG. 5. Consequently, a current begins to flow in the load circuit viathe capacitor C5, the capacitor C6 and the inductor L3. The currentI_(ST2), increasing in absolute terms, flows against the voltagedirection U_(CA,T2) defined in FIG. 2, and therefore has a negativesign. The negative voltage pulse at the gate electrode simultaneouslycauses the rise of a control current I_(L2) through the inductor L2,which for the most part once again reverses the charge of the two inputcapacitances C_(E,T1), C_(E,T2) of the transistors T1, T2. The currentI_(L2) opposes the voltage supplied by the diac such that the previouslynegative gate voltage is reduced.

[0031] With reference to FIG. 5, the start of the period 4 c is typifiedin that after the threshold voltage of the transistor T2 is undershotthe latter blows, and the load current injected in the inductor L3 nowflows via the resistor R5 and discharges the capacitor C4. At the sametime as the change in the voltage across the capacitor C4, there is alsoa change in the voltage across the Miller capacitances C_(M,T1),C_(M,T2) of the two transistors T1, T2. In particular, as a result ofthis the tie point of the two source electrodes is positioned virtuallyat the voltage U_(Z), the Miller capacitances C_(M,T1), C_(M,T2)participating fully in this voltage excursion. The change in voltage atthe Miller capacitances C_(M,T1), C_(M,T2) effects a current flow inaccordance with $I_{c} = {C \cdot {\frac{{U_{c}(t)}}{t}.}}$

[0032] These charging and discharging currents I_(CM,T1), I_(CM,T2)effect an additional current flow through the inductor L2. In thisphase, energy is fed to the control circuit via the Miller capacitancesC_(M,T1), C_(M,T2).

[0033] The start of the period 4 d is typified in that following thedrop in the drain-source voltage across the transistor T1 the innerfreewheeling diode thereof turns on, and thus takes over the main partof the current of the inductor L3. As soon as the majority of the loadcurrent is flowing via the freewheeling diode of the transistor T1, seeI_(ST1) in FIG. 5, the charge-reversal processes of the Millercapacitances C_(M,T1), C_(M,T2) are largely concluded. The currentinjected in the inductor L2 now chiefly reverses the charge of the twoinput capacitances C_(E,T1), C_(E,T2) of the two transistors T1, T2, andfirst effects a further drop in the gate voltage to zero and finally arise to positive voltage values. After the threshold voltage of thetransistor T1 is exceeded, the latter is turned on. The current injectedin the inductor L2 decays to zero during the on phase of T1, the startof period 4 e being defined thereby. The input capacitances C_(E,T1) andC_(E,T2) of the two transistors T1, T2 are charged appropriately suchthat the current can now flow from the charged capacitances to the gatecircuit L2, R4 (just as in the load circuit, in the control circuit theresult is also a current reversal). The control current I_(L2) throughthe inductor L2, which is now flowing in the reverse direction,counteracts the still positive gate voltage. The gate voltage, whichtherefore decreases, finally undershoots the threshold voltage of thetransistor T1, such that the latter turns off. This is the start ofperiod 4 f of FIG. 5. The current injected in the inductor L3 now flowsagain via the resistor R5 and charges the capacitor C4. At the same timeas the change in the voltage across the capacitor C4, there is also achange in the voltage across the Miller capacitances C_(M,T1), C_(M,T2)of the two transistors T1, T2. The charging and discharging currentsI_(CM,T1), I_(CM,T2) thereby constrained again effect an additionalcurrent flow through the inductor L2, see the lowermost diagram in FIG.5. In this phase, energy is fed to the control circuit once more via theMiller capacitances C_(M,T1), C_(M,T2). After a rise in the voltage viathe Miller capacitance C_(M,T2) of the transistor T2, the internalfreewheeling diode of the transistor T2 turns on and takes over the mainpart of the current of the inductor L3, see the start of period 4 g. Assoon as the majority of the load current is flowing via the freewheelingdiode of the transistor T2, the Miller charge-reversal processes arelargely concluded. The current IL2 injected in the inductor L2 chieflyreverses the charges of the two input capacitances C_(E,T1), C_(E,T2) ofthe transistors T1, T2, and initially effects a further drop in the gatevoltage to zero and, finally, a rise to negative voltage values.

[0034] The switching processes of periods 4 b to 4 g in FIGS. 4 and 5proceed further sequentially such that a current flow oscillating to andfro is produced in the control circuit and in the load circuit. Thefrequency of this initial oscillation is somewhat higher than theresonant frequency of the load circuit. Finally, owing to resonancestep-up the voltage across the illuminating means La rises so stronglythat the latter ignites. The majority of the load current is now flowingvia the illuminating means La. After ignition, the illuminating meansitself acts virtually like a purely ohmic consumer, and damps theresonant circuit in such a way that the voltage required for the stableoperation is finally set up across the illuminating means La.

[0035] The innovation of the present invention consists in principle infeeding so much energy to the control resonant circuit in periods 4 cand 4 f owing to charging and discharging processes of the Millercapacitances C_(M,T1), C_(M,T2) of the two switching elements T1, T2that an oscillation is maintained.

1. A free-running circuit arrangement for operating a load (La), havingat least one switching element (T1, T2), a freewheeling diode that isconnected in an anti-parallel fashion relative to the main currentdirection of the at least one switching element (T1, T2), a load circuit(L3, C6, C5) and a control resonant circuit (L2, C_(E,T1), C_(E,T2))which comprises at least one control inductor (L2) and at least oneself-capacitance (C_(E,T1), C_(E,T2)) of the at least one switchingelement (T1, T2), characterized in that the at least one switchingelement (T1, T2) has a control electrode, a working electrode and areference electrode, there acting between the control electrode and theworking electrode a capacitance (C_(M,T1), C_(M,T2)) that is coupled tothe control resonant circuit (L2, C_(E,T1), C_(E,T2)) in such a way thatenergy is fed into the control resonant circuit (L2, C_(E,T1), C_(E,T2))by the charging and discharging current (I_(CM,T1), I_(CM,T2)) of thiscapacitance (C_(M,T1), C_(M,T2)), the circuit arrangement having nocomponents for feeding energy into the control resonant circuit byelectromagnetic coupling.
 2. The circuit arrangement as claimed in claim1, characterized in that the at least one switching element (T1, T2) hasa Miller capacitance (C_(M,T1), C_(M,T2)), and the capacitance actingbetween the control electrode and the working electrode of the switchingelement (T1, T2) comprises the Miller capacitance (C_(M,T1), C_(M,T2))of the at least one switching element (T1, T2).
 3. The circuitarrangement as claimed in claim 2, characterized in that the controlresonant circuit (L2, C_(E,T1), C_(E,T2)) and the Miller capacitance(C_(M,T1), C_(M,T2)) of the at least one switching element (T1, T2) aretuned to one another in such a way that the oscillation of the controlresonant circuit (L2, C_(E,T1), C_(E,T2)) is maintained solely by thecharging and discharging currents (I_(CM,T1), I_(CM,T2)) of the Millercapacitance (C_(M,T1), C_(M,T2)) of the at least one switching element(T1, T2).
 4. The circuit arrangement as claimed in claim 2,characterized in that an additional capacitance (C_(MZ)) is connected inparallel with the Miller capacitance (C_(M,T1), C_(M,T2)) of the atleast one switching element (T1, T2), the oscillation of the controlresonant circuit (L2, C_(E,T1), C_(E,T2)) being maintained by thecharging and discharging currents (I_(CM,T1), I_(CM,T2)) of the Millercapacitance and of the additional capacitance (C_(MZ)) of the at leastone switching element.
 5. The circuit arrangement as claimed in claim 1,characterized in that the self-capacitance of the at least one switchingelement is a self-capacitance (C_(E,T1), C_(E,T2)) of the latter presentbetween control and reference electrodes.
 6. The circuit arrangement asclaimed in claim 1, characterized in that it has a first and a secondswitching element (T1, T2), the first and the second switching elements(T1, T2) being of complementary design and being coupled to a commoncontrol resonant circuit (L2, C_(E,T1), C_(E,T2)).
 7. The circuitarrangement as claimed in claim 6, characterized in that each switchingelement (T1, T2) has a control electrode, a working electrode and areference electrode, the control electrodes being connected to oneanother with the formation of a first tie point, and the referenceelectrodes being connected to one another with the formation of a secondtie point, the control inductor (L2) being coupled between the first andthe second tie point.
 8. The circuit arrangement as claimed in claims 1,characterized in that it has a first and a second switching element (T1,T2) that are of the same type, the first switching element (T1) beingcoupled to a first control resonant circuit, and the second switchingelement (T2) being coupled to a second control resonant circuit.
 9. Thecircuit arrangement as claimed in claim 8, characterized in that eachswitching element (T1, T2) has a control electrode, a working electrodeand a reference electrode, the reference electrode of the firstswitching element (T1) being connected to the working electrode of thesecond switching element (T2), and the respective control inductor beingcoupled between the respective control and reference electrodes of therespective switching element.
 10. The circuit arrangement as claimed inone of claims 6 to 9, characterized in that the first and the secondswitching element (T1, T2) are arranged in a half-bridge arrangement.11. The circuit arrangement as claimed in claim 1, characterized in thatthe load is an illuminating means (La), preferably a low-pressuredischarge lamp.
 12. The circuit arrangement as claimed in claim 5,characterized in that a supplementary capacitance (C_(EZ)) is connectedin parallel with a input capacitance (C_(E,T1), C_(E,T2)) of the atleast one switching element (T1, T2).
 13. The circuit arrangement asclaimed in claim 1, characterized in that the load circuit comprises aseries tuned circuit with an inductor (L3), a capacitance (C6) connectedin parallel with the load and at least one decoupling capacitance (C5).14. The circuit arrangement as claimed in claim 1, characterized in thatthe at least one switching element (T1, T2) is an MOS field effecttransistor, and the freewheeling diode connected in an anti-parallelfashion to the at least one switching element (T1, T2) is the body diodeof the latter.